1. Field of the Invention
This invention relates generally to a corona igniter for emitting a radio frequency electric field to ionize a fuel-air mixture and provide a corona discharge, and a method of forming the igniter.
2. Description of the Prior Art
Corona discharge ignition systems include an igniter with a central electrode charged to a high radio frequency voltage potential, creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as a non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. Preferably, the electric field is controlled so that the fuel-air mixture does not lose all dielectric properties, which would create a thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter. An example of a corona discharge ignition system is disclosed in U.S. Pat. No. 6,883,507 to Freen.
The corona igniter typically includes the central electrode formed of an electrically conductive material for receiving the high radio frequency voltage and emitting the radio frequency electric field to ionize the fuel-air mixture and provide the corona discharge. The igniter also includes a shell formed of a metal material receiving the central electrode and extending longitudinally from an upper shell end to a lower shell end. An insulator formed of an electrically insulating material is disposed in the shell and surrounds the central electrode. The igniter of the corona discharge ignition system does not include any grounded electrode element intentionally placed in close proximity to a firing end of the central electrode. Rather, the ground is preferably provided by cylinder walls or a piston of the ignition system. An example of a corona igniter is disclosed in U.S. Patent Application Publication No. 2010/0083942 to Lykowski and Hampton.
During operation of the corona igniter, when the central electrode is at a maximum possible positive voltage, such as a 100% voltage, and the shell is grounded at the lowest possible voltage, such as a 0% voltage, an ionized gas is formed in a gap between the insulator and the shell. Under certain conditions, a very high electric field strength exists in the gap. Negative ions of the ionized gas typically follow a voltage potential gradient and electric field over the surface of the insulator to the central electrode, forming a conductive path from the shell to the central electrode. The ionized gas is also formed in a gap between the central electrode and insulator, and an identical situation exists, except with the charges, voltages, and currents reversed. The conductive path between the central electrode and shell can create undesirable power-arcing and deplete the remaining corona discharge, which can degrade the quality of ignition.